The present invention relates to an interpolation method for a numerical control machine.
When a workpiece is machined spirally with a numerical control machine, that is, when it is machined along a spiral segment which, as shown in FIG. 3, has a start point S.sub.0 on a straight axis X passing through the center 0 of a rotational axis and a finish point S.sub.F shifted by a pitch P in the X-axis direction every revolution, linear interpolation should be effected along the X-axis and the C-axis simultaneously. In this case, the machining speed v is: EQU v=.omega..multidot.r, (1)
where .omega. is the angular speed around the C-axis and r is the radius.
The machining speed v should be constant. However, in practice, since it is required that the angular speed .omega. be constant, the machining speed v increases as the radius r increases. This means that the method is employable only in the case where the difference between the radii of the start point S.sub.0 and the finish point S.sub.F of the spiral segment is not large. Therefore, when, in the case where the difference between the radii of the start point and the finish point of a spiral segment is relatively large and a uniform cutting operation is required, a method should be employed whereby the spiral segment is divided into a plurality of blocks S.sub.0 S.sub.1, S.sub.1 S.sub.2, . . . and S.sub.n-1 S.sub.n so that the machining speed is not high. The machining operation is carried out with these blocks subjects to X-axis and C-axis simultaneous linear interpolation.
In order to machine a workpiece along the blocks S.sub.0 S.sub.1 and S.sub.1 S.sub.2 at a speed v.sub.c, machining instructions should be issued so that anuglar speeds v.sub.c /r.sub.0 and v.sub.c /r.sub.1 are used for the blocks S.sub.0 S.sub.1 and S.sub.1 S.sub.2, respectively, where r.sub.0 is the radius of the point S.sub.0 of the block S.sub.0 S.sub.1 and r.sub.1 is the radius of the point S.sub.1 of the block S.sub.1 S.sub.2. That is, it is necessary to slightly change the speed around the C-axis for every block.
Thus, the conventional interpolation method is disadvantageous in that it is difficult to maintain the machining speed constant, with the result that the program for interpolation is considerably complex.